This invention relates to aqueous coating compositions based on amino acrylate polymers.
It is generally desirable to use water based coatings instead of organic solvent based coatings for the sake of environmental considerations. However, the use of water instead of organic solvents in coating compositions based on polyisocyanates containing free isocyanate groups has been hampered by the fact that isocyanate groups react not only with isocyanate-reactive groups in the intended crosslinking reaction, but also with the water. Therefore, in a composition including a polyisocyanate, isocyanate-reactive groups, and water, the isocyanate/water reaction takes place with formation of urea and carbon dioxide. This reaction not only fails to achieve the intended crosslinking of the isocyanate reactive groups, but also results in gassing or foaming of the composition due to the formation of carbon dioxide. Various attempts have been made in the prior art to stabilize aqueous isocyanate compositions against the undesired side reaction with water.
In U.S. Pat. No. 5,225,505 there are disclosed amino acrylate polymers produced by emulsion polymerization in water. This patent teaches that it is necessary to include acrylic acid as one of the monomers for the polymerization. The polymeric products of the procedure disclosed in that patent have been found to be unsuitable for use in coating compositions. The deficiencies are demonstrated in a replicated example set forth herein.
It was disclosed in U.S. Pat. No. 5,075,370 that it is possible to produce aqueous, two-component polyurethane coating compositions by using neutralized, acid group-containing polyhydroxy polymers, i.e. polyhydroxypolyacrylates, as dispersing agents for polyisocyanates containing free isocyanate groups. The polyisocyanates containing free isocyanate groups are emulsified in the aqueous, anionic polymer solution or dispersion. The coating compositions according to that patent are said to have a pot life of several hours and cure by way of isocyanate/hydroxyl group reaction to form polyurethane linkages.
In commonly owned, copending U.S. patent application Ser. No. 08/357,488 filed Dec. 16, 1994, there is disclosed and claimed two-component waterborne coating compositions based on cationic amino acrylate dispersions and isocyanates that are relatively free from side reactions with water and which react to form polyureas. The amino acrylate polymers are dispersed into water by neutralization with an acid, with the result that the coating compositions have pH below 7.0. These compositions are satisfactory for use on metals if applied over a previously applied primer coating. For use in direct contact with a metal substrate, however, coatings having pH below 7.0 have the drawback of promoting corrosion of the substrate. It would be desirable to have coatings of the type disclosed in the aforesaid patent application that have pit greater than 7.0, thereby rendering them useful for direct application onto metal substrates.